The present invention relates to an RF modulator for converting the video signal of a video tape recorder or the like into a signal suitable for a television or the like.
In a conventional video tape recorder (hereinbelow, abbreviated to "VTR"), video disk player (hereinbelow, abbreviated to "VD") and video camera (hereinbelow, abbreviated to "TV camera"), an RF modulator 17 as shown in FIG. 1 has been generally employed.
The RF modulator 17 is so constructed that a clamp circuit 1, a video amplifier circuit 2, a video modulation circuit 3, a carrier generator circuit 5, an audio amplifier circuit 9, an audio modulation circuit 7, an audio oscillator circuit 8, a frequency converter circuit 6 and a carrier change-over switch 16 are received in a shield case 17a. It operates so that a video signal supplied through a switching circuit 18 from a video signal source 12 in the VTR, VD, TV camera or the like and an audio signal supplied from an audio signal source 10 are converted into a suitable signal for a television, which is prescribed according to the local broadcasting standards.
The TV signal delivered from the RF signal output terminal 4 of the RF modulator 17 is supplied to, for example, the antenna terminal of a television receiver (not shown). In a device in which the carrier of the RF modulator 17 can be changed at will by means of the switch 16 or the like, a reference signal generator 11 for generating a specified video signal not used by the general broadcasting stations, i.e., a test signal, is disposed so that the TV signal transmitted from the VTR, VD or TV camera can be distinguished from a TV signal transmitted from a broadcasting station in the television receiver. Thus, the test signal from the reference signal generator 11 can be supplied to the RF modulator 17 on occasion by operating the switching circuit 18.
However, in the device in which the reference signal generator 11 and the switching circuit 18 are arranged outside the shield case 17a of the RF modulator 17 as in the prior art, an undesirable radiated interference from the reference signal generator 11 may develop. Another problem is that a signal from the carrier generator circuit 5 or the audio oscillator circuit 8 may propagate to a cable connecting the RF modulator 17 and the reference signal generator 11, so an undesirable radiated interference may develop.